[Tropical] Tropical Cyclone Discussion

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WTNT43 KNHC 200238
TCDAT3

Tropical Storm Bret Discussion Number   3
NWS National Hurricane Center Miami FL       AL032023
1100 PM AST Mon Jun 19 2023

This evening, Bret's structure on satellite has evolved from a
curved banding pattern to a growing central dense overcast near
the estimated center. The exact location of the center is tricky to
pinpoint though, most likely positioned just west of the deepest
central convection. Dvorak estimates from TAFB and SAB were both
T2.5/35-kt at 00 UTC, 37-kt from UW-CIMSS, and unfortunately there
have not been any helpful microwave or scatterometer passes this
evening. The initial intensity is being held at 35 kt for this
advisory in agreement with the majority of the available data.

The initial motion appears to be just north of due west at 275/16
kt. A deep-layer ridge that is currently positioned overhead of Bret
will initially follow along with the cyclone, maintaining a west or
north of due west heading for the next 48-72 hours. As the system
approaches the Lesser Antilles later this week, a mid- to
upper-level trough is forecast to amplify somewhat, with the
vertical depth of Bret likely to influence its future track. Once
again there is a large spread in both along and cross track model
predictions, with the ECMWF on the south and west end, and the GFS
and some of the strongest hurricane regional models (HWRF, HAFS-A)
on the north and east end. The consensus aids have shifted ever so
slightly south and west from the previous forecast cycle, and the
NHC track will follow suit, especially beyond 60-h. However, there
continues to be larger than normal uncertainty in the track forecast
given the large spread of the guidance suite.

In the short-term, environmental conditions remain quite favorable
for additional intensification, with shear remaining under 10-kt,
and ample mid-level moisture and anomalously warm SSTs maintaining a
favorable thermodynamic environment. Thus, steady intensification
seems likely for the next day or so. However, between 36-60 h,
mid-level moisture begins to markedly decrease along Bret's forecast
track, and both the GFS and ECMWF suggest mid-level northwesterly
shear may begin to undercut the more favorable upper-level
easterlies. This is where the intensity forecast becomes tricky,
since a more vertically coherent system could be more resilient
against these less favorable conditions versus one that remains less
aligned and more susceptible to dry air intrusion. The intensity
guidance also diverges over this time frame, with some of the
regional hurricane guidance showing continued intensification,
compared to leveling off from the global model guidance. The latest
NHC forecast is a bit more conservative at this time frame, but
still shows a peak intensity at hurricane intensity between 48-60 h.
Thereafter, even drier air and additional shear is likely to begin a
weakening trend, and the latest NHC intensity forecast shows a bit
more weakening than the previous cycle.


KEY MESSAGES:

1. Bret is forecast to initially strengthen and then move across
the Lesser Antilles near hurricane intensity on Thursday and
Friday, bringing a risk of flooding from heavy rainfall, strong
winds, and dangerous storm surge and waves.

2. Given the larger than usual uncertainty in the track forecast,
it is too early to specify the location and magnitude of where
these hazards could occur. However, everyone in the Lesser
Antilles, Puerto Rico, and the Virgin Islands should closely monitor
updates to the forecast for Bret and have their hurricane
plan in place.


FORECAST POSITIONS AND MAX WINDS

INIT  20/0300Z 11.4N  43.5W   35 KT  40 MPH
 12H  20/1200Z 11.8N  45.8W   45 KT  50 MPH
 24H  21/0000Z 12.3N  48.7W   55 KT  65 MPH
 36H  21/1200Z 12.8N  51.4W   60 KT  70 MPH
 48H  22/0000Z 13.3N  53.9W   65 KT  75 MPH
 60H  22/1200Z 13.8N  56.7W   65 KT  75 MPH
 72H  23/0000Z 14.3N  59.4W   60 KT  70 MPH
 96H  24/0000Z 15.0N  64.5W   55 KT  65 MPH
120H  25/0000Z 16.5N  69.5W   50 KT  60 MPH

$$
Forecaster Papin
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